The present invention relates to an improved system whereby the torque required to perform an electric start of a prime mover, such as a turbine engine, may be achieved with reduced current and reduced inverter power (kVA) rating, thereby reducing the cost, size and weight of the system. Specifically, the present invention provides an improved power electronics system in which starting torque is delivered by a salient pole synchronous machine while associated power electronics utilization is maximized by adjusting the Park vector of the armature phase current to remain in-phase with the Park vector of the armature terminal voltage within a synchronous machine.
In a traditional system of starting a prime mover, such as a gas turbine engine, a starter motor is used to apply torque to the turbine engine""s shaft for some period. As described in U.S. Pat. No. 6,035,626 issued to Wahl et al., the entire content of which being incorporated herein by reference, the gas turbine engine includes a compressor, a combustor, and a turbine. The compressor and the turbine are mounted for rotation on a shaft, which may also be used to drive other components, such as a gearbox and various accessories, such as an alternating current (AC) electric generator and lube pump.
As described in the Wahl Patent, as the shaft starts to rotate, air is inducted into the compressor, compressed and then discharged in the combustor. Concurrently, the engine""s fuel control system feeds fuel into the combustor in accordance with a preprogrammed fuel schedule to precisely maintain the proper fuel to air ratio in the combustor. At a rotational speed of about 10 to 20 percent of the engine""s operating speed, the conditions in the combustor become such that the fuel/air mixture can be ignited at a stage commonly known as xe2x80x9clight-offxe2x80x9d. Should the fuel to air ratio be either too rich or too lean, light-off will not occur and the engine will experience a xe2x80x9chung startxe2x80x9d. After light-off the starter motor torque is augmented by torque from the engine""s turbine. At about 50 percent of operating speed the starter motor is shut off. At this point, the engine is self-sustaining and accelerates itself to operating speed.
Typically, direct current (DC) motors are used as starter motors in such applications. However, as explained in the Wahl Patent, for a given power supply, DC motor torque-versus-speed characteristics are fixed. Consequently, a DC starter motor must be sized to produce starting torque under the worst condition with the greatest engine drag, which is typically a cold soaked engine. Such DC starter motors are of a type commonly referred to as brush-type motors, and have severe reliability problems due to the brush-commutator interface that can deteriorate rapidly due to high altitude starts. Further, the speed-torque profile of the motor is fixed and cannot be adjusted.
One solution to overcome the disadvantages associated with DC motors discussed in the Wahl Patent includes the use of the prime mover""s AC generator as an AC starter motor. Generators, in combination with power converters, are commonly referred to as starter/generators (S/Gs). Furthermore, as pointed out in the Wahl Patent, the power converter can regulate the power consumption of the starter/generator from either a DC or AC source, therefore the converter can be controlled to provide any desired starting torque characteristic or torque-versus-speed profile. Such systems generally require the use of power inverters that control the current applied to the stator winding of the main generator in addition to a low power inverter that supplies single phase excitation to the exciter field.
As known to those skilled in the art, a starter/generator synchronous machine is doubly excited, with electrical energy supplied to both the field and armature windings. A DC voltage is typically applied to the field winding located on the rotor, and an AC current is applied to the armature winding located on the stator. Where current exists in each winding, a torque is produced resulting from the alignment of associated magnetic fields. Ideally, torque maximization for a synchronous machine can be achieved by having the armature current Park vector in-phase with the back-emf vector, which is located in the q-axis. Additional details regarding such torque control are discussed in U.S. Pat. No. 5,818,192 issued to Farhad Nozari, the entire content of which being incorporated herein by reference. The Nozari Patent discloses a method of control which serves to optimize a starter/generator synchronous machine used in starter applications. The armature current vector of the synchronous machine in the Nozari Patent is controlled to remain in-phase with the back-emf vector, which optimizes the machine, but can not consistently optimize the control electronics, such as the synchronous machine inverter. Optimization of the electronics, primarily the inverter, is critical, as the cost per kVA rating is many times greater than for the synchronous machine.
In applications such as turbine starting, whether for the main engine or APU start, a minimum torque must be provided at or about xe2x80x9clight offxe2x80x9d of the engine in order to continue acceleration of the engine. This minimum torque must be provided by a vector product of main field flux and stator current vector. Optimizing the rating of the two inverters (main and field) would minimize the cost and weight of the combination. Saturation of the machine plays an important role in determining this optimum, since once the machine is saturated, further increase in field current does not increase the field flux appreciably.
Accordingly, a need exists for a starting system using an attached synchronous machine to accelerate a prime mover, such as a gas turbine engine, where control electronics are optimized while maintaining torque levels at or near maximum values by adjusting the armature current Park vector of the synchronous machine, particularly when the machine is saturated.
An object of the present invention is to provide torque, generated by a synchronous machine, for the start-up of an attached prime mover, such as a gas turbine engine.
Another object of the present invention is to provide and optimize a control system to direct the operation of the synchronous machine in the starting system.
Still another object of the present invention is to achieve torque levels at or near maximum values while maintaining optimization of the synchronous machine control electronics.
Still another object of the present invention is to achieve significant cost savings by optimizing control electronics used during start-up of an attached prime mover.
These and other objects are substantially achieved by providing a system and method for controlling the armature current of a starter/generator (S/G) during turbine engine start-up applications. The system and method controls the armature current Park vector of the S/G to remain in-phase the terminal voltage Park vector of the system thereby maintaining equal active power and apparent power levels, and maximizing the kVA rating of the electronics, primarily the S/G inverter. This strategy enables both the reluctance and reaction torque of the machine to be utilized, thereby reducing the required kVA rating of the inverters used for starting.